1. Field of the Invention
The present invention relates to a polyoxymethylene composition which is useful for producing molded articles having excellent weather resistance, and to a molded articles thereof. More particularly, the present invention relates to polyoxymethylene composition which, when molded, can be used as an exterior part without requiring the surface of the molded article to be plated or coated due to the weather-resistant characterization of the article and to a molded article obtained from the composition.
2. Discussion of Related Art
Polyoxymethylene is superior in mechanical strength, friction resistance, wear resistance, fatigue resistance, moldability, etc. and is in wide use in such fields as electronics, electricity, automobiles and precision machines.
Molded articles of polyoxymethylene exhibit poor weather resistance properties when used as a molded article for exterior parts (e.g. outside door handle of automobile). Molded articles of polyoxymethylene in general has insufficient resistance to weather conditions. Therefore, when a polyoxymethylene molded article is exposed to sunlight for a long period of time, the polyoxymethylene undergoes molecular fission due to the light energy at the surface of the molded article, and as a result, whitening and cracking takes place and the appearance of the molded article becomes substantially impaired. When these phenomena occur and are severe, the molded article will possess reduced mechanical strength.
In order to reduce these problems, there have been proposed, for example, a method wherein an ultraviolet absorber (e.g. benzophenone type ultraviolet absorber, benzotriazole type ultraviolet absorber and the like) is added to a polyoxymethylene molded article and a method wherein the above ultraviolet absorber and a hindered amine type light stabilizer are added in combination to a polyoxymethylene molded article.
However, the former methods of adding, an ultraviolet absorber or a mixture thereof with a hindered amine type light stabilizer, to a polyoxymethylene molded article, requires the addition of these additives in large amounts in order to obtain sufficient weather resistance. In these cases, deterioration would occur, for example, the heat stability, appearance and mechanical strength properties of the article, whereby the characteristics for the polyoxymethylene are impaired.
A method is presently in use in which a protective layer such as a plating layer, a coating layer or the like, is formed on the surface of a polyoxymethylene molded article to protect the surface.
However, even though a method of forming a protective layer (e.g. a plating layer, a coating layer) on the surface of a polyoxymethylene molded article is free from the above-mentioned problems, there is still the drawback of an increase in process steps, which in turn would increase production costs.